Chiral liquid crystal (LC) materials are useful for many applications, for example LC displays (LCD) or polymer films with a twisted structure. Usually they consist of an LC host material containing one or more chiral dopants which induce the desired helical twist. The effectiveness of a chiral compound to induce a helically twisted molecular structure in a liquid crystal host material is described by its so-called helical twisting power (HTP). The HTP is given in first approximation, which is sufficient for most practical applications, by equation (1):
                    HTP        =                  1                      p            ·            c                                              (        1        )            wherein c is the concentration of the chiral compound in the host material and p is the helical pitch.
As can be seen from equation (1), a short pitch can be achieved by using a high amount of the chiral compound or by using a chiral compound with a high absolute value of the HTP. Thus, in case chiral compounds with low HTP are used, high amounts are needed to induce a short pitch. This can be disadvantageous in case the chiral dopant has a negative influence on the properties of the LC host mixture, like clearing point, dielectric anisotropy, viscosity, driving voltage or switching times. Also, chiral dopants are typically used as pure enantiomers and can be expensive and difficult to synthesize.
A cheap, stable, chiral polymerisable dopant is an essential component for the mass production of cholesteric and other chiral optical polymer films made from reactive mesogens for display applications. A chiral dopant is also required for example in the mass production of security films made from reactive mesogens, and in LCD applications, like for example LCDs utilising the electro-optical blue phase as a switching medium, or surface-stabilised cholesteric texture (SSCT) displays.
However, many of the chiral dopants described in prior art are expensive because of the cost of the starting materials or the number of steps required for production of the final compound. In addition, many chiral dopants described in prior art do not have a very high twisting power (i.e. a high absolute value of the HTP), meaning that a large amount of the dopant is required to achieve an appropriate level of helicity and selective Bragg reflection, as shown above. This makes the LC material expensive.
Also, chiral dopants known from prior art often show low solubility in the LC host material, which leads to undesired crystallization at low temperatures. To overcome this disadvantage, typically two or more different chiral dopants have to be added to the host mixture. This implies higher costs and usually also requires additional effort for temperature compensation of the material, since the different dopants have to be selected such that their temperature coefficients of the twist will compensate each other.
Consequently, there is a considerable demand for chiral compounds which have a high twisting power, are easy to manufacture, can be used in low amounts, show low temperature dependence of the twisting power e.g. for utilizing a constant reflection wavelength, show good solubility in an LC host material and do not have a negative influence on the properties of the LC host.
The invention has the aim of providing chiral compounds having these properties, and not having the above-mentioned disadvantages of prior art chiral compounds. Another aim of the invention is to extend the pool of chiral compounds available to the expert. Other aims are immediately evident to the expert from the following description.
The inventors of the present invention have found that these aims can be achieved by providing chiral compounds as claimed in this invention, which are derived from substituted 1,1′-binaphthyl-2,2′-diyl sulfate.
1,1′-Binaphthyl-2,2′-diyl sulfate is disclosed in C. Koy et al., Sulfur Letters 1998, 21(2), 75-88. However, substituted binaphthyl sulfates as claimed in the present invention are not disclosed.